Dual hysteresis control circuit

ABSTRACT

A control circuit, in particular for a direct current control in positioning systems, comprising a differential circuit (1), a control logic (2) and a full bridge (3) connected between a supply voltage V S  and a reference potential GND. The differential circuit (1) has a first hysteresis comparator (HC1) and a second hysteresis comparator (HC2). The two comparator inputs (HC1-, HC1+, HC2-, HC2+) of the two hysteresis comparators (HC1, HC2) are connected each to one of two input terminals (IN1, IN2) of the control circuit and crosswise to a comparator input of the respective other comparator (HC1, HC2). The inverting input of each comparator (HC1, HC2) is connected to the non-inverting input of the respective other comparator. (FIG. 1)

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.08/335,809, filed Jan. 31, 1995, now U.S. Pat. No. 5,559,416.

The invention relates to a control circuit, in particular for a directcurrent motor control in positioning systems, according to the genericclause of patent claim 1.

Such a control circuit is known from the publication Siemens Components27 (1989), No. 2, pages 79 to 82.

Positioning systems equipped with direct current motors are frequentlyused for keeping a mechanical variable, e.g. a linear movement or atilting movement, in track with an adjustable electrical voltage. Suchpositioning systems may be used in an automobile for controlling theposition of air duct dampers, or the position of headlights tocompensate for variable load conditions. Motor positioning systems arealso used in computers in applications such as positioning a magnetichead on a hard disk drive.

FIG. 9 shows a control circuit for positioning systems with directcurrent motors according to the prior art. The direct current motor tobe controlled is connected between the output of a first operationalamplifier OP1 and the output of a second operational amplifier OP2. Avoltage divider constituted by a first voltage divider resistor R1, asecond voltage divider resistor R₂ and a third voltage divider resistorR₃ is connected between a voltage supply V_(S) and a reference potentialGND. The inverting input of the first operational amplifier OP1 isconnected to the connecting node between the second voltage dividerresistor R₂ and the third voltage divider resistor R3. The non-invertinginput of the second operational amplifier is connected to the connectingnode between the first voltage divider resistor R₁ and the secondvoltage divider resistor R₂. The non-inverting input of the firstoperational amplifier OP1 is connected to the inverting input of thesecond operational amplifier OP2; the two latter inputs are fed with aninput voltage V_(IN). Between the outputs of the two operationalamplifiers there is connected a direct current motor. Mechanicallycoupled therewith is a potentiometer R_(F) detecting the actual value ofthe rotational position of the motor. This actual value is fed back viaa feedback resistor R_(R) to an input of operational amplifier OP1. Thisinput furthermore is connected via a resistor R_(IN) to the tap of anominal value potentiometer R_(C).

FIG. 10 illustrates the transfer function of this known circuitaccording to FIG. 9. Plotted on the abscissa is the input voltageV_(IN), whereas the differential output voltage V_(OUT) is plotted onthe ordinate. When the input voltage V_(IN) is lower than V₁, thedifferential output voltage V_(OUT) is negative and in its amountvirtually corresponds to the supply voltage V_(S). The motor therebyrotates in a first direction, for example in a right-hand rotation. Whenthe input voltage V_(IN) is continuously increased until it exceeds athreshold value V₁ corresponding to the potential at the connecting sitebetween resistors R₂ and R₃, the differential output voltage V_(OUT)changes to the value of zero volt. The motor stops. When upon furtherincrease of the input voltage a threshold value V₂ is exceeded, whichcorresponds to the potential at the connecting site between resistors R₁and R₂, the differential output voltage V_(OUT) increases to a positivevalue which virtually corresponds to the amount of the supply voltageV_(S). The motor then rotates in a second direction, namely in aleft-hand rotation in the example assumed.

A problem of the circuit shown in FIG. 9 is that it displays a linearbehavior in the range of the threshold values V₁, V₂ and in practicalrealization therefore requires an output compensation circuit (Boucherotmember). In addition thereto, this circuit involves a sensitive reactionto interference voltages superimposed on the input voltage, especiallywhen V_(IN) after stopping of the motor is close to one of thethresholds V₁ and V₂.

It is desirable to have a control circuit which involves hysteresis inthe switching thresholds V₁ and V₂ and furthermore is adapted to processalso differential signals.

Such a control circuit suitable for positioning control, e.g. withdirect current motors, has a transfer characteristic as shown in FIG.11. According to this figure, there are three different startingconditions: positive output voltage causing e.g. a left-hand motorrotation; output voltage zero, at which the motor is short-circuited andwhich thus means a stop condition; negative output voltage causingright-hand motor rotation in the particular example. The input voltagesare the nominal and actual voltages of the positioning system. The inputvoltage difference V_(dIN) represents the difference between thesevoltages.

The function of the control circuit and the insensitiveness tointerferences of the positioning system may be elucidated from the shapeof the transfer characteristic indicated in FIG. 11. With a negativeinput voltage difference V_(dIN) <V_(dn), a negative voltage is appliedto the motor, which rotates in right-hand direction. The input voltagedifference is increased thereby; when the threshold V_(dp-) is reached,the output voltage of the comparator circuit becomes zero and the motoris stopped in the position reached. The positioning accuracy isdetermined by the difference V_(dP) reached between the nominal andactual voltages.

    V.sub.dp- ≦V.sub.dP ≦V.sub.dp+.

Upon reaching of the stop condition, the motor remains decelerated aslong as V_(dIN) has not exceeded one of the threshold values V_(dn-) orV_(dn+). The distance between the thresholds V_(dn-) and V_(dp-) on theone hand and the thresholds V_(dn+) and V_(dp+) (hysteresis) on theother hand determines the insensitiveness of the positioning system tointerferences. An interference voltage with an amplitude of up to V_(dN)≦V_(dN+) -V_(dp+), which is superimposed on the actual or the nominalinput voltage, can be handled without having an effect on the motorposition reached when the positive portion of V_(dIN) is considered. Thesame applies to the negative portion when it is assumed that thebehavior with respect to offset and hysteresis is the same in thenegative portion and in the positive portion.

It is thus an object of the invention to make available a controlcircuit according to the generic clause of the main claim, which can beintegrated in inexpensive manner while displaying the desired transferbehavior according to FIG. 11.

This object is met by the features indicated in patent claim 1 and maybe developed in advantageous manner according to the dependent claims.

The circuit according to the invention comprises an analog differentialinput having a large common mode region. The input offset voltagesnecessary for realizing the desired transfer characteristic aregenerated internally in the differential input stages of the hysteresiscomparators. The desired transfer characteristic according to FIG. 11 isrealized by the wiring of the comparator inputs and the adaptation ofthe transfer function of the further circuit parts.

The advantages achieved by the invention in particular consist in thatthey make available a control circuit, in particular for a directcurrent motor control in positioning systems, which may also be employedin an environment affected by electromagnetic interference fields, forexample in a motor vehicle. In addition thereto, a control circuit, inparticular for direct current motor control in positioning systems, ismade available which can be realized with few components, in particularalso in the form of an integrated circuit (IC).

Further advantages and developments are gatherable from the embodiments.

A preferred embodiment of the invention makes use of a hysteresiscomparator in which both the hysteresis values and the offset values canbe adjusted without any problem while requiring very few component partsand thus little chip area in the case of monolithic integration.

The reference Siemens Components 27 (1989), No. 2, pages 79 to 82,reveals an integrated driver circuit for a full bridge current supplyfor electric motors which is free from transverse currents and in whicheach half bridge is digitally controlled via a control input of its own,and a separate control branch extends from each control input to therespective associated half bridge, with each control input beingfollowed by a hysteresis element of its own in the form of a Schmitttrigger for definitely generating a digital control signal for thebridge circuit. However, the hysteresis elements do not constitute adifferential circuit. This means, on the control input side there isprovided one input Schmitt trigger for each one of two control signalinputs, in order to achieve an as definite switching behavior aspossible also in case of digital input signals affected byinterferences. The two control signal inputs serve for separatelysupplying control signals for control of the one and the other bridgebranch, respectively, of a full bridge circuit driving a direct currentmotor. However, there is no differential circuit operation involved, asit is essential for the control circuit according to the presentinvention.

In the control circuit for a motor control clocked for four quadrants,as known from the publication EPE Proceedings, 1991, Vol. 3, pages 3-562to 3-567, each comparator has a separate reference voltage source of itsown associated therewith. The reference voltages are dimensioned suchthat the desired four quadrant behavior can be obtained. An offset ispreset for each of the four comparators from the outside via itsassociated reference voltage source. The hysteresis of these comparatorsdetermines the current stroke of the clocked current control carriedout.

The invention will now be elucidated by way of several embodiments. Inthe drawings

FIG. 1 shows a circuit diagram of a general embodiment of a controlcircuit according to the invention;

FIG. 2 shows transfer functions in connection with a first embodiment(FIG. 2A) and a second embodiment (FIG. 2B), respectively, of thesolution according to the invention;

FIG. 3 shows a basic circuit diagram of a first embodiment of a controlcircuit of the invention according to FIG. 1;

FIG. 4 shows a basic circuit diagram of a control logic and a fullbridge of a second embodiment of a control circuit of the inventionaccording to FIG. 1;

FIG. 5 shows a circuit diagram of a first application of a controlcircuit according to the invention;

FIG. 6 shows a circuit diagram of a second application of a controlcircuit according to the invention;

FIG. 7 shows a circuit diagram illustrating an embodiment of the basicstructure of each one of the hysteresis comparators in FIGS. 1 and 4;

FIG. 8 shows a circuit diagram illustrating details of the hysteresiscomparator shown in FIG. 7;

FIG. 9 shows a positioning system comprising a direct current motoraccording to the prior art;

FIG. 10 shows a transfer function of the known circuit according to FIG.9; and

FIG. 11 shows a transfer function of known type with hysteresisbehavior.

FIG. 11 shows a diagram of a known-per-se transfer function withhysteresis behavior, which constitutes a basis for the invention.Plotted on the ordinate is the difference V_(M) of the output voltagesV_(OUT1) and V_(OUT2) of a control circuit as a function of thedifference V_(dIN) of the input voltages IN1 and IN2 of a controlcircuit, which is plotted on the abscissa. At a value of thedifferential voltage V_(dIN) =0 V, the difference V_(M) of the outputvoltage V_(M) =0 V. When the differential voltage V_(dIN) is graduallyincreased starting from zero volt, the difference V_(M) of the outputvoltages at a predetermined value V_(dIN) =V_(dn+) increases from V_(M)=0 V to a positive voltage level V⁺. When the differential voltage.V_(dIN) is then gradually decreased again, the difference V_(M) of theoutput voltages, at a value V_(dIN) =V_(dp+), jumps from V_(M) =V⁺ tozero volt. When the differential voltage V_(dIN) is gradually decreasedstarting from zero volt, the difference V_(M) of the output voltages, ata value V_(dIN) =V_(dn-), jumps from V_(M) =0 V to a negative voltagelevel V⁻. When the differential voltage V_(dIN) is then graduallyincreased again, the difference V_(M) of the output voltages, at apredetermined value V_(dIN) =V_(dp-), jumps from V_(M) =V⁻ to zero volt.

In the following, the function of a control circuit with theafore-described transfer function shall be described in a positioningsystem. At a negative differential voltage V_(dIN), with V_(dIN) withV_(dIN) <V_(dn-), the motor is thus acted upon with a negative voltageand rotates, for example, towards the right. In so doing, thedifferential voltage V_(dIN) increases. When the threshold V_(dp-) isreached, the output voltage becomes zero, and the motor is stopped inthe position reached. This condition is maintained until thedifferential voltage V_(dIN) passes one of the threshold values V_(dn-)or V_(dn+). The distance between the thresholds V_(dn-) and V_(dp-) onthe one hand and V_(dn+) and V_(dp+) on the other hand (hysteresis)determines the insensitiveness of the system to interferences. Aninterference voltage V_(dN) superimposed on the input voltage and havingan amplitude of V_(dN) <V_(dn+) -V_(dp+) can be handled without havingan effect on the motor position. The same applies again in correspondingmanner for the negative portion of V_(dIN).

For the voltage interval between the values V_(dp-) and V_(dp+) for thedifferential voltage V_(dIN), the direct current motor to be controlledis short-circuited in order to reduce after-running.

FIG. 1 shows a circuit diagram of a general embodiment of a controlcircuit 100 according to the invention, comprising a differentialcircuit 1, a control logic 2 and a full bridge 3. A first analog inputIN1 of control circuit 100 is connected directly to a first differentialinput HC1₋ of a first hysteresis comparator HC1 as well as to a seconddifferential input HC2₊ of a second hysteresis comparator HC2 ofdifferential circuit 1. A second analog input IN2 of control circuit 100is connected directly to a second differential input HC1₊ of the firsthysteresis comparator HC1 as well as to a first differential input HC2₋of the second hysteresis comparator HC2. The control logic 2 having afirst input LDI₁, a second input LDI₂, a first output LDO₁, a secondoutput LDO₂, a third output LDO₃, and a fourth output LDO₄ is followedby the full bridge 3 connected between a supply voltage V_(S) and areference potential GND and consisting of a first half-bridge with firstand second switch means SW11, SW12 controllable via one control inputeach, as well as a second half-bridge with third and fourth switch meansSW21, SW22 controllable via one control input each, with an output HC1₀of the first hysteresis comparator HC1 being connected directly to thefirst logic input LDI₁ of control logic 2. In corresponding manner, anoutput HC2₀ of the second hysteresis comparator HC2 is connecteddirectly to the second logic input LDI₂ of control logic 2. Furthermore,the first, second, third, and fourth outputs LDO₁, LDO₂, LDO₃, LDO₄ ofcontrol logic 2 are each connected to the control input of said first,second, third, and fourth switch means SW11, SW12, SW21, SW22,respectively.

A first control circuit output OUT1 is connected to the bridge tap ofthe first half bridge SW11, SW12, and a second control circuit outputOUT2 is connected to the bridge tap of the second half bridge SW21,SW22. Each of the controllable switch means SW11, SW12, SW21, SW22 isadapted to have its switching path controlled into a conducting or ablocking switching state, depending on whether the associated controlinput is at a first or a second potential.

The comparator circuit 1 may be designed for different transferfunctions depending on the hysteresis switching behavior of each of thetwo hysteresis comparators HC1 and HC2 as well as the cooperationthereof. FIGS. 2A and 2B illustrate two different examples of suchtransfer functions. The transfer function of hysteresis comparator HC1is shown in each figure in full lines, whereas the transfer function ofhysteresis comparator HC2 is shown in each figure in broken lines. Thefigures each show the output voltage V_(HC10) of hysteresis comparatorHC1 and the output voltage V_(HC20) of hysteresis comparator HC2,respectively, as a function of the input voltage V_(dIN).

The different transfer functions according to FIGS. 2A and 2B arebrought about when the connection system on the input side of thecomparator circuit 1 is changed with respect to the connection systemdepicted in FIG. 1. FIG. 2A shows the transfer function of thecomparator circuit 1 according to FIG. 1. When the two inputs areexchanged in the two hysteresis comparators HC1, HC2, a transferfunction according to FIG. 2B is created.

The different transfer characteristics of differently designedcomparator circuits 1 cause embodiments of the output circuitconstituted by control logic 2 and full bridge 3 which have differenteffects. FIGS. 2A and 2B each have a transfer function associatedtherewith, which indicates the transfer behavior of this output circuitwhich is to be associated with each one of the transfer functions shownin FIGS. 2A and 2B so that a motor connected to the output terminalsOUT1 and OUT2 shows the same behavior in all embodiments. The symbolsused in the various transfer function tables have the followingmeanings:

1: a first potential or current condition

0: a second potential or current condition

TS: a tri-state condition (high-impedance output)

-: motor rotation in a first direction

+: motor rotation in a second direction of rotation

FIG. 3 shows a detailed circuit diagram of a first embodiment of acontrol circuit of the invention according to FIG. 1, realizing atransfer behavior according to FIG. 2A. The connection of the hysteresiscomparators HC1 and HC2 corresponds to the arrangement already shown inFIG. 1 and elucidated hereinbefore; reference is made to thesestatements. The full bridge 3 shown in FIG. 1 as an aggregate ofcontrollable switch means SW11, SW12, SW21, SW22, as well as theassociated control logic 2 are realized in a preferred embodiment interms of circuit design as shown in FIG. 3. Corresponding to the first,second, third, and fourth controllable switch means SW11, SW12, SW21,SW22 in FIG. 1 is a first, second, third, and fourth npn switchingtransistor T2, T4, T6, T8, respectively, with the switching path of eachswitch means implemented in the form of a transistor being designed eachas a collector-emitter path of the respective transistor. The associatedbase serves as a control input for the respective transistor. Thecontrol logic 2 is implemented by a first pnp multicollector transistorT1, a second pnp multicollector transistor T5 as well as a first npnauxiliary transistor T3 and a second npn auxiliary transistor T7. Thecollector-emitter path of first npn auxiliary transistor T3 is connectedin parallel to the base-emitter path of second switching transistor T4.The collector-emitter path of second npn auxiliary transistor T7 isconnected in parallel to the base-emitter path of the fourth switchingtransistor T8. The base of first switching transistor T2 is connected toa first collector of the first multicollector transistor T1. A secondcollector of first multicollector transistor T1 is connected to the baseof first auxiliary transistor T3. The base of fourth switchingtransistor T8 is connected to a third collector of the firstmulticollector transistor T1. In corresponding manner, the base of thirdswitching transistor T6 is connected to a first collector of the secondmulticollector transistor T5. A second collector of secondmulticollector transistor T5 is connected to the base of secondauxiliary transistor T7. The base of second switching transistor T4 isconnected to a third collector of the second multicollector transistorT5.

The related transfer function table is shown beside FIG. 2A.

The circuit shown in FIG. 3, just as the embodiments elucidatedhereinafter, may in principle also be composed with field effecttransistors.

FIG. 4 shows a detailed block diagram of the control logic 2 and thefull bridge 3 of a second embodiment of a control circuit of theinvention according to FIG. 1, realizing a transfer behavior of thetransfer function table relating to FIG. 2B. The connection of thehysteresis comparators HC1 and HC2 corresponds to the arrangementalready shown in FIG. 1 and elucidated hereinbefore; reference is madeto these statements. The full bridge shown in FIG. 1 as an aggregate ofcontrollable switch means SW11, SW12, SW21, SW22, as well as theassociated control logic 2 are realized in a preferred embodiment interms of circuit design as shown in FIG. 4. Corresponding to the first,second, third, and fourth controllable switch means SW11, SW12, SW21,SW22 in FIG. 1 is a first, second, third, and fourth npn switchingtransistor T2, T4, T6, T8, respectively, with the switching path of eachswitch means implemented in the form of a transistor being designed eachas a collector-emitter path of the respective transistor. The associatedbase serves as a control input for the respective transistor. Thecontrol logic 2 is implemented by a first pnp multicollector transistorT1, a second pnp multicollector transistor T5, a first npn auxiliarytransistor T3, a second npn auxiliary transistor T7, a third npnauxiliary transistor T9, a fourth npn auxiliary transistor T10, and afifth npn auxiliary transistor T11. The collector-emitter path of firstnpn-auxiliary transistor T3 is connected in parallel to the base-emitterpath of second switching transistor T4. The collector-emitter path ofsecond npn auxiliary transistor T7 is connected in parallel to thebase-emitter path of the fourth switching transistor T8. The base offirst switching transistor T2 is connected to a first collector of thefirst multicollector transistor T1. The base of third switchingtransistor T6 is connected to a first collector of the secondmulticollector transistor T5. The base of second switching transistor T4is connected to the emitter of the fourth npn auxiliary transistor T10.The base of fourth switching transistor T8 is connected to the emitterof the third npn auxiliary transistor T9. A second collector terminal offirst multicollector transistor T1 as well as the base thereof areconnected to the collector of the third npn auxiliary transistor T9. Incorresponding manner, a first collector terminal as well as the baseterminal of second multicollector transistor T5 are connected to thecollector of the fourth npn auxiliary transistor T10. The base of firstnpn auxiliary transistor T3 is connected to the emitter of the sixth npnauxiliary transistor T12. The table beside FIG. 2B shows the truth tableof control logic 2 of the second embodiment of a control circuitaccording to the invention.

FIG. 5 shows a circuit diagram of a first application of the generalembodiment of a control circuit according to the invention as shown inFIG. 1. First analog input IN1 of control circuit 100 is connected via afirst preresistor R_(IN-) to the sliding contact of a firstpotentiometer R_(C) whose end terminals are connected via a secondpreresistor R_(C1) and third preresistor R_(C2), respectively, to avoltage source BAT and to a reference potential GND, respectively.Furthermore, the supply voltage V_(BAT) of voltage source BAT issupplied to the control circuit 100 in usual manner via a terminal VS.Finally, the control circuit is connected to the reference potential viaa terminal GND. A direct current motor M to be controlled ismechanically coupled with a second potentiometer R_(F) connected betweenthe supply voltage V_(BAT) and the reference potential GND, so that apredetermined position of the sliding contact of the secondpotentiometer R_(F) corresponds to each mechanical operating conditionof the system to be positioned. The sliding contact tap of the secondpotentiometer R_(F) is connected to the second control circuit input IN2via a fourth preresistor R_(IN+).

FIG. 6 shows a circuit diagram of a second application of the embodimentof a control circuit of the invention according to FIG. 1. The firstanalog input IN1 of control circuit 100 is connected via a firstpreresistor R_(IN-) to the center tap of a voltage divider R₃, R₄connected between the supply voltage V_(BAT) and the reference potentialGND. The second analog input IN2 of control circuit 100 is connected viaa fifth preresistor R_(INC) to the sliding contact of a firstpotentiometer R_(C), whose end terminals are again connected via asecond preresistor R_(C1) and third preresistor R_(C2), respectively, tothe voltage source BAT and to the reference potential GND, respectively.Moreover, the supply voltage V_(BAT) of the voltage source BAT is fed tothe control circuit 100 in usual manner via a terminal VS. Finally, thecontrol circuit is connected to the reference potential via a terminalGND. A direct current motor M to be controlled is mechanically coupledwith a second potentiometer R_(F) connected between supply voltageV_(BAT) and reference potential GND, so that a predetermined position ofthe sliding contact of the second potentiometer R_(F) corresponds toeach mechanical operating condition of the system to be positioned. Thesliding contact tap of the second potentiometer R_(F) is connected tothe second control circuit input IN2 via a fourth preresistor R_(INF).

FIG. 7 shows a circuit diagram illustrating an embodiment of the basicconstruction of each hysteresis comparator HC1 and HC2, respectively, inFIG. 1. The circuit shown in FIG. 7 consists of an asymmetricalemitter-coupled differential amplifier (differential input stage) I_(b),R₁₀₁, T₁₀₁, T₁₀₂ comprising a controllable current mirror CM and adecoupling transistor T₁₀₅ for coupling out the current mirror outputsignal onto a comparator output HCOUT and for controlling a currentmirror switching operation.

The current ratio k of current mirror CM is switchable between twovalues k₀ and k₁. The input switching threshold hysteresis is achievedby switching over of the current ratio of the current mirror CM of thedifferential input stage in response to the initial state of this stage.The amount of the input switching threshold hysteresis is determined bythe value of the biasing current (I_(b)) and by the difference of thetwo current ratios k₀ and k₁ of current mirror CM.

The input difference offset voltage is reached by an asymmetricalemitter negative feedback of transistors T101, T102 by means of resistorR101. With this asymmetrical negative feedback, an input differenceoffset voltage results which is a function of the biasing current andthe current ratio k₀ and which is generated by the voltage drop atresistor R101.

FIG. 8 shows an embodiment of the hysteresis comparator HC1 or HC2,respectively, illustrated in FIG. 7 in its basic form. The circuit shownin FIG. 8 consists of an asymmetrical emitter-coupled differentialamplifier I_(b), R₁₀₁, T₁₀₁, T₁₀₂ comprising a controllable currentmirror T₁₀₃, T₁₀₄, T₁₀₆ and a decoupling transistor T₁₀₅ which effectscontrol of the current mirror switching operation, as well as a currentmirror output transistor T107.

In the embodiment shown in FIG. 7, the decoupling transistor T₁₀₅ mayalso be omitted when the circuit is designed in corresponding manner. Inthis case, current mirror switching is effected directly with an outputsignal HCOUT.

The controllable current mirror CM depicted in FIG. 8 comprises a firstnpn current mirror transistor T₁₀₃ and a second npn current mirrortransistor T₁₀₄ which is composed as a multiemitter transistor. Thecollector and the base of the first current mirror transistor T₁₀₃ areshort-circuited with each other. The emitter of the first current mirrortransistor T₁₀₃ as well as a first emitter of the second current mirrortransistor T₁₀₄ are applied to reference potential GND. A second emitterof the second current mirror transistor T₁₀₄ is connected to thecollector of a third npn current mirror transistor T₁₀₆. The emitter ofthe third current mirror transistor T₁₀₆ is applied to referencepotential GND. The collector of the current mirror control transistorT₁₀₅ is connected both via an output stage preresistor R₁₀₃ to the baseof the current mirror output transistor T₁₀₇ and via a switchingpreresistor R₁₀₂ to the base of the third current mirror transistorT₁₀₆. The base of the current mirror control transistor T₁₀₅ isconnected to the collector of the second current mirror transistor T₁₀₄,while the emitter thereof is applied to ground potential GND.

For reasons of circuit design, a resistor may also be connected into theemitter path of transistor T₁₀₁. In this case the difference between thetwo emitter resistors is determinative for the input difference offsetvoltage and the input switching threshold hysteresis.

The function of this embodiment is based on the fact that each one ofthe hysteresis comparators HC1, HC2 comprises a switchable currentmirror stage CM having a first operating state (current ratio k₀) and asecond operating state (current ratio k₁), with the current mirror stageCM changing its operating state in response to the output signal of thehysteresis comparator HC1, HC2, and that the current mirror stage CMcomprises a multi-emitter transistor T₁₀₄, with the second emitterterminal of the multi-emitter transistor T₁₀₄ being connected to groundin response to the output signal of transistor T₁₀₆.

The offset of the switching thresholds is determined by the dimensionschosen for the current source I_(b), the current ratio k₀ of currentmirror CM and the resistor R₁₀₁, whereas the hysteresis is determined bythe biasing current (I_(b)), the difference of the currents at the twocurrent ratios (k₀, k₁) of current mirror CM and by resistor R₁₀₁. Theratio of the current mirror currents may be fixed by selectingcorresponding dimensions for the emitter areas of the emitters oftransistors T₁₀₃ and T₁₀₄.

The described embodiment of the comparator circuit provides thepossibility of varying, by changing of the biasing current I_(b), at thesame time offset (V_(dp-), V_(dp+)) and hysteresis (V_(dn-) -V_(dp-),V_(dn+) -V_(dp+)). Upon application in particular in a positioningsystem, this property provides the possibility of realizing both thenoise suppression range and the positioning range of the control systemas a function of the supply voltage and thus of achieving a positioningaccuracy that is unaffected by the supply voltage. With increasingsupply voltage the noise suppression range is thereby equally increasedas the interference voltages and the interference immunity reserveremains thus unaffected by the supply voltage.

I claim:
 1. A control circuit for controlling a direct current motorhaving first and second motor terminals, the circuit having a firstsupply terminal coupled to a power supply, and a second supply terminalcoupled to a reference voltage, first and second signal sourcessupplying first and second variable analog signals, respectively, thecircuit comprising:a differential input stage having first and secondinput terminals coupled to the first and second signal sources,respectively, and receiving a differential input signal corresponding tothe difference between the first and second variable analog signals, thedifferential input stage further having first and second differentialoutput terminals each generating an output signal having first andsecond voltage states and having a hysteresis effect wherein said firstoutput differential terminal changes from said first voltage state tosaid second voltage state when said differential input signal exceeds afirst predetermined threshold level and changes from said second voltagestate to said first voltage state when said differential input signal isless than a second predetermined threshold level different from saidfirst predetermined threshold level and said second differential outputterminal changes from said first voltage state to said second voltagestate when said differential input signal is less than a thirdpredetermined threshold level and changes from said second voltage stateto said first voltage state when said differential input signal exceedsa fourth predetermined threshold level different from said thirdpredetermined threshold level, said differential input stage comprisingfirst and second hysteresis comparators, each having a positive andnegative input terminal, said positive input terminal of said firsthysteresis comparator and said negative input terminal of said secondhysteresis comparator both being coupled to said first input terminal,and said negative input terminal of said first hysteresis comparator andsaid positive input terminal of said second hysteresis comparator bothbeing coupled to said second input terminal; a control stage having afirst and second control input terminals coupled to said first andsecond differential output terminals, said control stage having first,second, third, and fourth control output terminals generating first,second, third, and fourth control output signals, respectively; and afull bridge switching stage coupled to the first and second supplyterminals and controlled by said first, second, third, and fourthcontrol output signals, said full bridge switching stage having firstand second motor output terminals coupled to the first and second motorterminals, respectively, and controlling the direct current motor byselectively coupling the first and second motor terminals to the powersupply and the reference voltage.
 2. An automobile electric motorcontrol circuit, the circuit comprising a battery having supply voltagecoupled to a first supply terminal on the circuit and a referencevoltage coupled to a second supply terminal on the circuit;first andsecond signal sources supplying first and second variable analogsignals, respectively; a direct current motor having a first and secondmotor terminals, said direct current motor having a positioncontrollable by the control circuit; a differential input stage havingfirst and second input terminals coupled to the first and second signalsources, respectively, and receiving a differential input signal andcorresponding to the difference between the first and second variableanalog signals, the differential input stage further having an outputterminal generating an output signal having first and second voltagestates and having a hysteresis effect wherein said output terminalchanges from said first voltage state to said second voltage state whensaid differential input signal exceeds a first predetermined thresholdlevel and changes from said second voltage state to said first voltagestate when said differential input signal is less than a secondpredetermined threshold level different from said first predeterminedthreshold level, said differential input stage comprising first andsecond hysteresis comparators, each having a positive and negative inputterminal, said positive input terminal of said first hysteresiscomparator and said negative input terminal of said second hysteresiscomparator both being coupled to said first input terminal, and saidnegative input terminal of said first hysteresis comparator and saidpositive input terminal of said second hysteresis comparator both beingcoupled to said second input terminal; a control stage having a controlinput terminal coupled to said output terminal, said control stagehaving first, second, third, and fourth control output terminalsgenerating first, second, third, and fourth control output signals,respectively; and a full bridge switching stage coupled to said firstand second supply terminals and controlled by said first, second, third,and fourth control output signals, said full bridge switching stagehaving first and second motor output terminals coupled to said first andsecond motor terminals, respectively, and controlling said position byselectively coupling said first and second motor terminals to said powersupply and said reference voltage.
 3. A motor control circuit for acomputer disk drive, the circuit comprising:a power supply having supplyvoltage coupled to a first supply terminal on the circuit and areference voltage coupled to a second supply terminal on the circuit;first and second signal sources supplying first and second variableanalog signals, respectively; a computer disk drive for driving amagnetic disk having electromagnetic media on at least a first surfaceto store data; a direct current motor having first and second motorterminals, said direct current motor having a motor positioncontrollable by the control circuit; a magnetic head coupled to saidmotor and positioned in proximity with said electromagnetic media ofsaid magnetic disk when disposed at the computer disk drive to read saidstored data from said electromagnetic media and to generate signals towrite data to said electromagnetic media, said motor position beingselected to permit said magnetic head to move to a selected location inproximity with said electromagnetic media to read said stored data fromsaid electromagnetic media at said selected location and to generatesignals to write data to said electromagnetic media at said selectedlocation; a differential input stage having first and second inputterminals coupled to the first and second signal sources, respectively,and receiving an a differential input signal and corresponding to thedifference between the first and second variable analog signals, thedifferential input stage further having first and second differentialoutput terminals each generating an output signal having first andsecond voltage states and having a hysteresis effect wherein said firstoutput differential terminal changes from said first voltage state tosaid second voltage state when said differential input signal exceeds afirst predetermined threshold level and changes from said second voltagestate to said first voltage state when said differential input signal isless than a second predetermined threshold level different from saidfirst predetermined threshold level and said second differential outputterminal changes from said first voltage state to said second voltagestate when said differential input signal is less than a thirdpredetermined threshold level and changes from said second voltage stateto said first voltage state when said differential input signal exceedsa fourth predetermined threshold level different from said thirdpredetermined threshold level, said differential input stage comprisingfirst and second hysteresis comparators, each having a positive andnegative input terminal, said positive input terminal of said firsthysteresis comparator and said negative input terminal of said secondhysteresis comparator both being coupled to said first input terminal,and said negative input terminal of said first hysteresis comparator andsaid positive input terminal of said second hysteresis comparator bothbeing coupled to said second input terminal; a control stage having afirst and second control input terminals coupled to said first andsecond differential output terminals, said control stage having first,second, third, and fourth control output terminals generating first,second, third, and fourth control output signals, respectively; and afull bridge switching stage coupled to said first and second supplyterminals and controlled by said first, second, third, and fourthcontrol output signals, said full bridge switching stage having firstand second motor output terminals coupled to said first and second motorterminals, respectively, and controlling said position by selectivelycoupling said first and second motor terminals to said power supply andsaid reference voltage.
 4. A motor control circuit having hysteresis tocontrol a motor having motor terminals, the circuit comprising:first andsecond signal sources supplying first and second variable analog controlsignals, respectively; first and second input terminals coupled to thefirst and second signal sources, respectively, to receive a differentialinput control signal corresponding to the difference between the firstand second variable analog control signals; a differential input stagecoupled to said first and second input terminals to receive saiddifferential input control signal, said differential input stagecomprising first and second hysteresis comparators, each having positiveand negative inputs to receive said differential input control signal,said positive input of said first hysteresis comparator and saidnegative input of said second hysteresis comparator both being coupledto said first input terminal and said negative input of said firsthysteresis comparator and said positive input of said second hysteresiscomparator both being coupled to said second input terminal, saiddifferential input stage generating an output signal having first andsecond levels in response to said differential input control signal,said output terminal changing from said first level to said second levelwhen said differential input control signal exceeds a first thresholdlevel and changing from said second level when said differential inputcontrol signal is less than a second threshold level different from saidfirst threshold level to create a hysteresis effect; and a control stagecoupled to said differential input stage to receive said output signaland to generate motor control signals in response thereto, said motorcontrol signals being coupled to the motor terminals, whereby the motoris controlled by said differential input control signal using thehysteresis effect.
 5. A motor control circuit of claim 4 to control saidmotor to a desired nominal position, wherein the first variable analogcontrol signal is a nominal voltage determined to control said motorinto said nominal position and the second variable analog control signalis an actual voltage indicating the actual position of said motor.
 6. Adual-hysteresis circuit coupled to first and second signal sourcessupplying first and second variable control signals, respectively, forgenerating output control signals, the circuit comprising:first andsecond input terminals coupled to the first and second signal sources,respectively, to receive a differential input control signalcorresponding to the difference between the first and second variablecontrol signals; a first hysteresis comparator having positive andnegative inputs to receive said differential input control signal, saidpositive input of said first hysteresis comparator being coupled to saidfirst input terminal and said negative input of said first hysteresiscomparator being coupled to said second input terminal, said firsthysteresis comparator generating a first output signal having first andsecond levels in response to said differential input control signal,said first output signal changing from said first level to said secondlevel when said differential input control signal exceeds a firstthreshold level and changing from said second level to said first levelwhen said differential input control signal is less than a secondthreshold level different from said first threshold level to create afirst hysteresis effect; a second hysteresis comparator having positiveand negative inputs to receive said differential input control signal,said positive input of said second hysteresis comparator being coupledto said second input terminal and said negative input of said secondhysteresis comparator being coupled to said first input terminal, saidsecond hysteresis comparator generating a second output signal havingthird and fourth levels in response to said differential input controlsignal, said second output signal changing from said third level to saidfourth level when said differential input control signal exceeds a thirdthreshold level and changing from said fourth level to said third levelwhen said differential input control signal is less than a fourththreshold level different from said third threshold level to create asecond hysteresis effect; a control stage coupled to said differentialinput stage to receive said first and second output signals and togenerate output control signals in response thereto; and output controlterminals to receive said output control signals, whereby the outputcontrol signals are controlled by a differential input control signalusing the hysteresis effect.
 7. A circuit of claim 6 wherein said firstlevel is substantially equal to said fourth level and said second levelis substantially equal to said third level.
 8. The circuit of claim 5for use with a motor having motor terminals coupled to said outputterminals to position the motor to a desired nominal position, whereinthe first variable control signal is a nominal voltage determined tocontrol the motor into said nominal position and the second variablecontrol signal is an actual voltage indicating the actual position ofthe motor.